The primary and obvious difference between journaling and blogging is that a journal is private. A journal exists only for the author’s personal consumption, or possibly as a posthumous record of a life. Without the modest audience my blog has accrued, I have no incentive to filter what I write for content or style. I can be as dry or as flowery as I like and nobody need suffer.
I've been doing this too, and went through some kind of dry spell in blogging in the process of working it out.
It seems like by blogging in 2009 you speak to a much larger audience, or a different kind of audience, to that you might have reached in say 2001. I remember one friend then writing very personal content in a blog, hidden in html comments. Was it to filter it to more technical readers, or to people looking closely rather than just skimming? Or was it, I speculate, a kind of desire to work out the boundary between the public and personal.
Beyond keeping a private diary, sometimes online and sometimes on paper, I've found it useful to keep special journals, append-only and dated, just in text files, on particular topics or projects. If I come back to a project after a gap of a few weeks or even a few minutes it reminds me where I was, and seems to help in reestablishing flow. And if it's the kind of issue that takes long-term contemplation it's quite enlightening to see what I thought of it six months ago. One can easily believe one always thought what one thinks now.